THE PHYSICAL BASIS OF MENDELISM 371 



however, the separation of the members of the chromosome pairs 

 results in sperms half of which contain an X-chromosome and half a 

 Y-chromosome in addition to the three autosomes. The reduction 

 division in the male insures an equality in numbers for the two kinds 

 of sperm cells and the chances that either kind of sperm will fertilize 

 an egg-cell are equal. By this arrangement the numerical equaUty 

 of the sexes is maintained. When, later, the egg cells of the female are 

 fertilized by the sperm cells of the male, as shown in the lower portion 

 of the figure, half of them being fertilized by sperm cells which contain 

 an X-chromosome will give females, and half uniting with sperm cells 

 which contain Y-chromosomes will produce males. The inheritance 

 of sex in Drosophila provides a beautiful illustration of the parallel 

 behavior of the chromosome mechanism and a somatic difference, in 

 this case, sex. 



To recapitulate, the essential phenomena of cell behavior which fur- 

 nish the mechanism for the distribution of hereditary factors are these: 



1. Every species is characterized by a definitely organized group 

 of chromosomes. The chromosomes occur in pairs, in each of which 

 one member is derived from each parent. In ordinary somatic mitosis 

 the distribution of chromatin is such that each daughter cell receives 

 a full complem.ent of chromosomes which are equivalent qualitatively 

 to those of the mother cell. 



2. In germ cell formation the homologous chromosomes conjugate 

 during synapsis, then separate, and pass into a division figure in which 

 entire homologous chromosomes are opposed to each other. The 

 resulting reduction division gives daughter cells with half the number 

 of chromosomes characteristic of the species, the half number being 

 made up of one member of each pair of chromosomes. During synap- 

 sis there is an opportunity for the members of a pair of chromosomes 

 to exchange chromatin material. WHien such interchange takes place 

 equivalent portions of chromosomes both quaHtatively and quantita- 

 tively are involved. In the reduction division segregation within one 

 pair of chromosomes is entirely independent of that of any other pair 

 so that the combinations of parental chromosomes in the germ cell? 

 represent all those to be expected on the basis of chance distribution. 



The student should constantly endeavor to harmonize this con- 

 ception of the distributing mechanism of the chromatin material with 

 the Mendelian interpretations of hereditary phemomena which will be 

 presented in what follows, to the end that he may obtain a clear and 

 definite idea of the interrelations between the known facts of heredity 

 and cell behavior. 



